How to Add Resistors to LED Lights

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Light emitting diodes are a way to provide low voltage lighting in confined spaces where the heat from conventional incandescent bulbs would be inappropriate. LEDs offer almost no electrical resistance and require a tiny operating current. Supplying excess voltage to an LED results in high currents that quickly burn out the LED. When an LED runs from a voltage higher than its normal operating voltage, you need a resistor to "burn off" the excess voltage and thus limit the current flowing through the LED.

Things You'll Need

  • Calculator
  • Emery paper
  • Soldering iron
  • Solder
  • Read the accompanying technical data sheet for the LED to find its forward voltage (Vf). Different LEDs run at different voltages. Blue and white LEDS have a typical Vf of 4 volts, and other LED colors are usually around 2 volts.

  • Deduct the LED's forward voltage from the power supply voltage to calculate the excess voltage. For example, the excess voltage for an LED with a Vf of 2 volts powered by a 9-volt battery is (9-2) volts, or 7 volts.

  • Read the LED's technical data sheet to establish the recommended current. A typical value is between 0.01 amps and 0.02 amps. Calculate the value for the resistor using the following formula: resistance equals excess voltage divided by current. For an LED with a current value of 0.02 amps and an excess voltage of 7 volts, the required resistor is 7/0.02 ohms, or 350 ohms. The 350-ohm resistors are not available, so use the next highest available value, 390 ohms. Resistors are available from any electrical component store and may be purchased online or salvaged from old appliances.

  • Rub the wires on the resistor with emery paper to remove dirt and oxidized metal. Melt a blob of solder on the tip of a low-wattage soldering iron. Rest the end of the positive leg of the LED on the solder blob. Add more solder until it's coated, and then remove it from the iron. Repeat this with the resistor wire. Bring the wire and LED leg together on the tip of the iron. Allow the solder to melt and join them. Let the joint cool without disturbance. Solder the resistor to the the positive lead from the power supply in the same manner.

Tips & Warnings

  • To be certain of the voltage across the LED use a digital multimeter to check the actual voltage. On 12-volt systems, use 12-volt LEDs with built-in resistors.
  • LEDs are polarized and will not illuminate if incorrectly wired. Excessive heat from prolonged soldering will destroy an LED.

References

  • Photo Credit Stockbyte/Stockbyte/Getty Images
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